CU-Boulder researchers shed light on faint sun 'paradox'

An artist s conception of the Earth during the late Archean, 2.8 billion years ago. Weak solar radiation requires the Earth have increased greenhouse gas amounts to remain warm. CU-Boulder doctoral student Eric Wolf and CU-Boulder Professor Brian Toon use a three-dimensional climate model to show that the late Archean may have maintained large areas of liquid surface water despite a relatively weak greenhouse. With carbon dioxide levels within constraints deduced from ancient soils, the late Archean may have had large polar ice caps but lower latitudes would have remained temperate and thus hospitable to life. The addition of methane allows the late Archean to warmed to present day mean surface temperatures. (Charlie Meeks / Courtesy University of Colorado)

Researchers at the University of Colorado believe they have taken significant strides toward solving one of the enduring mysteries of science: How did life exist on the ancient Earth at a time when the sun was much cooler than it is today?

That mystery is of sufficient duration that it has a name -- the faint young sun paradox.

And, as with more than a few problems in contemporary science, a solution might now be much closer thanks to far more powerful computing powers. In this case, much credit is being given to CU's Janus supercomputer, which has facilitated three-dimensional climate modeling of a younger Earth that is far more sophisticated than anything produced through more crude one-dimensional modeling in the past.

"Without that, it would have taken me 30 years of running this on a desktop computer, rather than six-and-a-half," said Eric Wolf, a CU doctoral student in the university's Atmospheric and Oceanic Sciences department, and lead author of a study appearing in the July issue of Astrobiology. "It's a tremendous resource."

Wolf co-authored the study with his doctoral adviser Brian Toon. It was funded by two NASA grants and by the National Science Foundation, which supports CU's Janus supercomputer.

Through their study, the researchers have concluded all that may have been required to sustain liquid water and primitive life on Earth during the Archean eon, about 2.8 billion years ago, were reasonable atmospheric carbon dioxide amounts believed to be present at the time, and perhaps a relatively small presence of methane. At that time, the sun was about 20 percent cooler than it is now.

"If you turn back the clock in geology, the sun was much less bright in the distant past than it is today, but we have geologic evidence for both liquid water and early life," Wolf said. "This poses an inconsistency off the top, of a very cold sun and a warm, habitable surface.

"The paradox side of it is that for many, many years, scientists have had trouble solving this problem... When we upgrade to a three-dimensional model, the paradox isn't as strong or as difficult to overcome as we might once have thought."

Toon, who along with Wolf is associated with CU's Laboratory for Atmospheric and Space Physics, was traveling Friday and could not be reached for comment.

But in a news release, he stated, "In our opinion, the one-dimensional models of early Earth created by scientists to solve this paradox are too simple -- they are essentially taking the early Earth and reducing it to a single column atmospheric profile. One-dimensional models are simply too crude to give an accurate picture."

Wolf and Toon utilized what is known as a general circulation model called the Community Atmospheric Model version 3.0 developed by the National Center for Atmospheric Research in Boulder, which contains 3-D atmosphere, ocean, land, cloud and sea ice components. The two researchers also "tuned up" the model with a sophisticated radiative transfer component that allowed for the absorption, emission and scattering of solar energy and an accurate calculation of the greenhouse effect for the unusual atmosphere of early Earth, which lacked both oxygen and ozone.

A solution to the faint sun paradox advanced by the researchers, which duplicates Earth's present climate, envisions a planetary atmosphere featuring roughly 15,000 parts per million of the greenhouse gas CO2 and 1,000 ppm of methane in the ancient atmosphere some 2.8 billion years ago, Wolf said.

That's far greater than the 400 ppm of CO2 in today's atmosphere, but geological studies of ancient soil samples support the idea that CO2 likely could have been that high during that time period, the CU scientists said. Methane is considered to be at least 20 times more powerful as a greenhouse gas than CO2, and could have played a significant role in warming the early Earth as well, the CU researchers said.

Wolf said his paper is the first in a wave of related studies expected to follow soon. He said the view advanced by he and Toon is by no means going to be the final word on the puzzle that was first addressed in 1972 by Cornell University scientist Carl Sagan -- Toon's doctoral adviser at the time -- and colleague George Mullen.

"I hesitate to say that the problem is solved, everybody go home, because surely there are always revisionists in science and even with very complex climate models... there's always a critic," Wolf said. "So I won't stand up and say I've solved everything."

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